This invention relates to a document of value such as a paper of value or ID card with a security element having an optically variable material that conveys different color effects at different viewing angles. The invention further relates to a foil, a multilayer transfer material and a printing ink with such an optically variable material.
In view of the high quality of color copies it is necessary to provide documents of value such as bank notes, ID cards or the like with additional security elements that are uncopiable or not copiable true to the original. For this purpose one frequently uses optically variable devices such as holograms, interference layer elements or liquid-crystal materials that convey different color effects at different viewing angles. The interplay of colors of said devices constitutes a very striking, visually easily recognized security feature so that a photocopy not having said interplay of colors can be distinguished from an original document very easily even by a layman. However, optically variable devices are accessible to machine testing only under certain conditions and with high technical effort.
EP 0 435 029 B1 describes a document of value with an optically variable, liquid-crystal security feature. One uses as security elements in particular liquid-crystal polymers that constitute a plastic-like solid with a distinctive color change at room temperature after suitably oriented production. They can be used to produce different types of security elements. One can thus coat plastic foils with a layer of liquid-crystal polymers and then cut the resultant sheet of material into narrow bands or threads that can be embedded in paper or other materials as security threads. It is also proposed in EP 0 435 029 B1 to produce transfer bands containing in their layer structure a layer of liquid-crystal polymers. Besides optically variable properties, liquid-crystal materials also possess light-polarizing properties that are exploited to make the security element additionally accessible to machine testing. Since light polarization is only a weak effect that is greatly impaired by environmental influences such as dirt, folds or bends in the document of value, however, the measuring effort for reliable automated testing is very high.